As a method for calculating an average of DC fed from DC power supply, a conventionally well-known method is disclosed in Japanese Patent Unexamined Publication No. H07-67248 (see P. 5, FIGS. 1 and 2). In the method, a current sensor for detecting DC is disposed on the power-supply line from DC power supply to an inverter and the integral of the detected current is obtained by a resistor and a capacitor.
FIG. 23 is a circuit diagram showing a conventional inverter and the peripheral circuits thereof. According to an rpm instruction signal (not shown) and the like, control circuit 108 of inverter 121 effects control of switching elements 2 of inverter circuit 37 so that DC voltage from battery 1 is switched by pulse-width modulation (PWM). Through the modulation, alternating current (AC) is fed to stator winding 28 of motor 30, and rotor 29 outputs power. In the explanations given hereinafter, it will be assumed that switching elements 2 are formed of upper-arm switching elements U, V, W and lower-arm switching elements X, Y, Z. Switching elements 2 are formed of a transistor, an IGBT (insulated gate bipolar transistor) and the like. Diodes 3 of inverter circuit 37 form a return route of current from stator winding 28.
Current sensor 6 is disposed between battery 1 and inverter circuit 37. Current sensor 6 detects a DC instantaneous value and sends it to operational amplifier 11. The DC instantaneous value is carried to control circuit 108 and is used for protecting switching elements 2. The value is also fed into an integrating circuit of resistor 12 and capacitor 13 and converted into an average value. Receiving the average value, control circuit 108 calculates the product of the average value and the voltage of battery 1 to obtain an input power for inverter 121. The input power for inverter 121 corresponds to power consumption of battery 1. Calculating the input power to be fed to inverter 121 is indispensable for monitoring and suppressing power consumption (i.e., electrical load) of battery 1.
On the other hand, detection of motor current (i.e., phase current) is well known as a method for controlling a motor with high accuracy and providing the motor with sine wave-shaped AC. Japanese Patent Unexamined Publication No. 2000-333465 (see P. 8, FIG. 1) discloses an example of the method, which will be described hereinafter.
FIG. 24 is a circuit diagram showing a conventional inverter and the peripheral circuits thereof. Current sensors 8 and 9 for detecting phase current are disposed between inverter circuit 37 and motor 31.
Control circuit 104 of inverter 120 receives a current value of U-phase from current sensor 8 and a current value of W-phase from current sensor 9. With the use of the two values obtained above, control circuit 104 calculates a current value of V-phase by Kirchhoff's current law applied at a neutral point of stator winding 28. According to the values for each phase, control circuit 104 further calculates induced voltage on stator winding 28 caused by magnet rotor 32 and determines the position of magnet rotor 32. According to an rpm instruction signal (not shown) and the like, control circuit 104 effects control of switching elements 2 of inverter circuit 37 so that DC voltage from battery 1 is switched by PWM. Through the modulation, sine wave-shaped AC is fed to stator winding 28.
As for the method for detecting phase current of a motor, Japanese Patent Unexamined Publication No. 2003-209976 introduces another example (see P. 21, FIG. 14).
FIG. 25 shows an inverter and the peripheral circuits thereof. A shunt for detecting phase current is disposed between the lower-arm switching elements and the battery 1; specifically, the circuit has shunt 15 disposed between ground and lower-arm switching element X for U-phase, shunt 16 disposed between the ground and lower-arm switching element Y for V-phase and shunt 17 disposed between the ground and lower-arm switching element Z for W-phase.
Control circuit 107 of inverter 122 calculates a current value of each phase according to the values of voltage from each shunt. According to the calculated current value, an rpm instruction signal (not shown) and the like, control circuit 107 effects control of switching elements 2 of inverter circuit 37 so that DC voltage from battery 1 is switched by PWM. Through the modulation, sine wave-shaped AC is fed to stator winding 28.
Operational amplifier 11 of the structure shown in FIG. 23 is not an absolute necessity. FIGS. 24 and 25 show the structures with no operational amplifier.
In the inverter having a current sensor between DC power supply (i.e., a battery) and an inverter circuit, to calculate an average value of DC fed from DC power supply, the structure requires—other than a current sensor and an operational amplifier—the following components: an integrating circuit formed of a resistor and a capacitor; an A/D port for receiving average current of a microprocessor in the control circuit. Each value obtained by the resistor and the capacitor of the integrating circuit has variations in resistance values, capacitance values and temperature change, which can invite poor accuracy in calculating an average value. To obtain higher accuracy, there is a need to know the correlation between the integral obtained by the integrating circuit and an actual average current. Besides, increase in parts count becomes an obstacle to downsizing and improvement of reliability.
In the inverter having a current sensor for detecting load current (i.e., phase current of a motor) between the inverter circuit and load (i.e., a motor), DC fed from DC power supply cannot be detected (, which means that average current also cannot be obtained). That is, the structure cannot calculate power consumption of DC power supply. From the detected phase current, the structure can calculate AC to be fed to the motor; in this case, there becomes a need to carry out additional computing, such as phase difference between current and voltage and calculation of PWM voltage. This places a significant computational burden on the microprocessor of the control circuit. Even if AC is calculated as a substitute for DC current, the calculated value has lack of accuracy since the power consumption of the inverter is not included.
Like in the inverter described above, the measurement of DC fed from DC power supply cannot be made by an inverter having a shunt for detecting phase current between the lower-arm switching elements and DC power supply (i.e., a battery). Therefore, the inverter also faces the aforementioned problems.